The present invention relates in general to the field of electrical power distribution systems. More particularly, the present invention relates to reclosers.
A common problem in almost any electrical power distribution system is a momentary disruption of electrical service, such as might be caused by a momentary short circuit. For example, power lines strung between poles could swing under wind loading, momentarily touching each other or a grounded conductor. Things may fall across exposed wires, arcing could occur, or other transitory events could cause momentary power line short circuits or current surges that could burn out a fuse or trip a recloser. Most of these faults are self-correcting and do not require permanent fuse or recloser protection because they terminate quickly. If a fuse should burn out or a recloser should trip, the power line would be open and customers would be deprived of their electrical power. Service calls to replace fuses or reset reclosers would then be required, thus escalating the utility""s costs.
A recloser is a fault-interrupting device used to sense current, voltage, and/or frequency and isolate faulted portions of distribution feeders. A recloser control device operates a recloser, which can be an electronic controller. Reclosers are inserted into power lines to protect a power distribution system.
More particularly, reclosers are electromechanical devices, similar to circuit breakers. Reclosers are distributed at one or more locations along a power line, typically upline from a fuse. When the recloser controller detects a fault condition, the recloser will begin to timeout. In other words, the recloser controller will initiate a trip to open the recloser if the fault condition has not cleared itself during a fixed time interval, where the time interval is a function of current. Then, after a time delay, as the name suggests, the recloser will close, and if the fault condition has been cleared, power service is restored. If, however, the fault condition has not been cleared, the recloser controller will again trip open the recloser after a second fixed time interval. If, after a predetermined number of reclose operations, the fault condition has not been cleared, the recloser controller will permanently lockout the recloser (i.e., permanently open the circuit). The circuit then remains open until the system is repaired and/or the fault condition is eliminated.
Thus, one primary function of a recloser is to save fuses. In general, this is done by sensing the peak value of the current conducted and interrupting its flow by opening or tripping a recloser before a fuse blows. After a time delay, the recloser closes, thereby restoring power to the system where it remains closed until the next fault is sensed.
The rate at which a fuse will blow and interrupt current is a function of the thermal heating of the fusible element. The rate of thermal heating is proportional to the power generated by the fault and each fuse has a time current characteristic, which describes the time interval required to interrupt the fault current. The time interval is generally approximately inversely proportional to the value of the root mean square of the fault current. It is desirable to coordinate the recloser with the fuses to be saved to insure that the recloser in fact interrupts temporary fault currents before the fuses to be protected are blown. This is generally done by approximating the root mean square value of the fault current by sensing its peak value.
It must also be recognized that not all faults, which occur on a power distribution line, are temporary, such as those caused by a tree branch momentarily falling against the line. Some faults are of a more permanent nature such as those caused by a storm where the entire pole structure has fallen to the ground. As a consequence, reclosers are built so that they will only trip a limited number of times within a short duration before locking open. Were this not done, a recloser would cycle until failure and many of the fuses to be protected would blow anyway.
At some magnitude of fault current it is desirable to have the recloser open immediately to protect the line rather than following an inverse time current characteristic. At intermediate levels it may be desirable from the power distribution standpoint to allow the fault current to flow for a limited period to allow the fault to burn itself open or blow the fuse. Many reclosers have alternate inverse time current characteristics, which achieve this goal. Typically, a recloser will allow two shots or trip operations to follow a fast time current characteristic and two additional shots along a somewhat slower time current characteristic before locking open or out.
Conventional reclosers for three-phase systems open all three phases at the same time upon detection of a fault on any one phase. Other three-phase systems implement three single-phase reclosers, one on each phase. In these systems, each single-phase recloser is independently controlled. This is costly and does not allow for the phases to be responsive to one another.
Thus, in a typical configuration, for a fault, the recloser will open to clear the fault. Note that for any fault, e.g. single-phase-to-ground faults, phase-to-phase faults, phase-to-phase-to-ground faults, and three-phase faults, a typical recloser will open all three phases. For distribution feeders, opening all three phases for a single-phase fault will result in more customers losing power than necessary. However, if an electric utility were to employ traditional single-phase recloser protection on their distribution system, it would be implemented with three independent single-phase mechanical reclosersxe2x80x94one for each phase. This provides a per phase approach to single-phase faults but if there is an evolving fault, such as, for example, two or more phases are faulted, then the tripping and subsequent reclosing is always done single-phase. Totally independent single-phase reclosers can also be involved in a race condition. For phase-to-phase faults, if one phase was to operate more quickly than the other does, the recloser may not correctly isolate the second faulted phase.
The recloser controller provides the intelligence that enables a recloser to sense overcurrent faults, select timing operations, time the tripping and reclosing functions, and lockout. The hydraulic unitxe2x80x94an integral part of the recloser is used in all single-phase reclosers and has a smaller rating of three-phase reclosers. The electronic controller is generally used in the single-phase reclosers and in higher ratings of three-phase reclosers. Such devices, however, should be appropriately programmed to coordinate in a predefined manner to endure that the power distribution systems respond to line faults in accordance with expectations.
Generally, when automatic circuit reclosers are used in conjunction with fuses, they are configured in a variety of modes. For example, the recloser or reclosing breaker may be configured for a fuse saving or fuse clearing mode. In the fuse saving mode, the automatic recloser or reclosing breaker operates faster than a fuse, trying to clear a momentary fault. If the fault is still present, the automatic circuit recloser operates more slowly than the fuse, enabling the fuse to blow and clear the fault. In the fuse-clearing mode, the automatic recloser is set so that for a fault beyond any fuse in series with the recloser, the fault shall be cleared by the fuse without causing the recloser to operate.
There are microprocessor-based recloser controllers which are capable of detecting fault conditions, and, in response, capable of timing out a corresponding recloser. However, these prior designs are not adaptive. Instead, fault detection in these prior designs is a function of some absolute (i.e., a fixed) preprogrammed current level, such that gradual changes in load current due to normal, daily and/or seasonal fluctuations are not taken into consideration. Therefore, service may be unnecessarily interrupted when the current fluctuation does not pose a threat to the system. Moreover, fault detection schemes based on an absolute current level may not always detect remote faults (i.e., faults that occur along a distal portion of a power line with respect to the position of the recloser).
It is thus desirable to provide recloser control that can overcome the problems of the prior art. In this way, an electric utility can adaptively set a recloser to function in a certain manner, such as fuse saving, fuse clearing, single-phase, or three-phase, based on prevailing conditions, such as time of day, day of week, month, or load current, for example.
The present invention is directed to recloser operation that is responsive to a particular protection setting group. Depending on the prevailing conditions, such as the time of day or the load current, the recloser functions in a certain manner, such as single-phase or in a fuse saving mode.
According to one embodiment within the scope of the present invention, a method for controlling a recloser for an electrical power line comprises determining a protection setting group, the protection setting group having at least one associated feature; determining a present condition of the at least one associated feature; determining a behavior function for the recloser based on the protection setting group and the present condition; and implementing the behavior function for the recloser, thereby controlling the recloser responsive to the behavior function.
According to aspects of the invention, the present condition is continuously monitored, and the behavior function is changed responsive to the monitoring. The monitoring can take place at predetermined times or events, such as predetermined intervals of time.
According to another embodiment within the scope of the invention, a recloser control system for an electrical power line comprises a recloser, a memory comprising a protection setting group having at least one behavior function with an associated feature, and a recloser controller coupled to the recloser and the memory for controlling the recloser responsive to one of the at least one behavior functions in the protection setting group.
The foregoing and other aspects of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.